Cleaning after dogs

I came to the conclusion that cleaning after one's dog's wrong. People get mad when you don't clean after your furry friend, but y? It doesn't make sense. There's a lot of advertisements about how eco life style is great. I do agree with that. But how cleaning shit after your dog is eco? You use to do it a plastic bag which you later throw out. To use the bag you had to buy it first. Creating such bags makes a lot of pollution. Why cleaning then? I mean- dog's shit is a great fertilizer, isn't it?